Cellular radio communications systems are widely used throughout the world to provide telecommunications to mobile users. A geographic area covered by a cellular radio system is divided into cells, each containing a cell site, through which subscriber units communicate. The geographic area is divided into cells for a number of reasons, including the following. First, bandwidth can be reused in different cells in order to meet the users' demand for communications capacity within an available frequency band allocation. Second, the use of cells limits the maximum transmission range required of a subscriber unit, thus limiting the power consumption of the subscriber unit. Third, the use of cells limits the number of subscriber units to be served by each cell site, which helps to maintain a satisfactory signal-to-interference ratio in each cell throughout the system.
In general, an object of cellular radio communications system design is to reduce the number of cell sites required by increasing their range and/or capacity. This is because cell sites are expensive, both because of the equipment required and because of the need for a geographical site for each cell site. Geographical sites may be costly and may require extensive effort to obtain planning permission. In some areas, suitable geographical sites may even not be available.
The communications ranges in many systems are uplink (mobile to cell site) limited because of the limited power available at the subscriber unit, which may be a hand-portable subscriber unit. However, any increase in range would mean that fewer cells would be required to cover a given geographical area, thus advantageously reducing the number of cell sites and associated infrastructure costs.
When a cellular radio system is set up in an area of high demand, such as a city, then cell site communications capacity, rather than range, usually limits cell size. An increased cell site capacity would therefore reduce the required number of cell sites and so reduce costs.
One approach to increasing range and/or capacity is to use directional antennas at a cell site physically to separate radiations at similar frequencies. This is known as sectorisation. It has been proposed to use three-sectored cells, having three antennas with nominally 120.degree. azimuthal beamwidth, or hex-sectored cells, having six antennas with nominally 60.degree. azimuthal beamwidth (as described for example in U.S. Pat. No. 5,576,717). In each case, one effect of the sectorisation is to reduce interference from mobiles and cell sites in adjacent and nearby cells, and thus to increase the total range and/or capacity of the cell site in a sectored cell relative to a cell using an omni-directional antenna.
However, there are problems which arise from the sectoring approach, particularly as the number of sectors increases. In any cellular system, a subscriber unit may move from one cell to another, necessitating transfer of the communication link from one cell site to another by a process known as handoff. In a sectored cell, a subscriber unit may also move from one sector to another, necessitating additional handoffs between the sectors of a cell site. Clearly, as the number of sectors increases, so does the number of handoffs, making increasing demands on the processing and communications capacity of the system.
One mode of communication used in cellular radio systems in which sectorisation may be particularly advantageous is spread spectrum communication, such as code division multiple access (CDMA). In such systems, all cell site transmissions, both in different sectors and in different cells, may be in the same frequency band.